题目内容：
根据下面资料，回答题 A Thirsty World
The world is not only hungry; it is also thirsty for water. This may seem strange to you sincenearly 75% of the earth' s surface is 31 with water. But about 97% of this huge amount is seawater,or salty water. Man can only drink and use the other 3% --the 32 water that comes fromrivers, lakes, underground, and other sources. And we cannot even use all of that, because some of itis in the form of icebergs and glaciers. Even 33 , some of it has been polluted.
At the moment, this small amount of fresh water is still enough for us. However, our demand forwater is increasing 34 . Only if we take steps to deal with this problem now can we avoid a severeworldwide water 35 later on.
In addition to stopping wasting our precious water, one useful step we should take is to 36 ways of reusing it.
Today, in most large cities, water is used only 37 and it eventually returns to the sea orruns into underground storage tanks, But it is 38 to pipe water that has been used to a purifying(净化) plant, There it can be filtered and treated with chemicals so that it can be used again just asif it were fresh from a spring.
But even if every large city purified and 39 its water, we still would not have enough.Where could we turn next? To the oceans! We can make use of the vast amount of sea-water by removing the salt. This salt-removing 40 is already in use in many parts of the world.
第（31）题选
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